This invention relates to a binderless ceramic or ceramic oxide hollow body and a method for its manufacture.
Ceramic or ceramic oxide hollow bodies are used for calcining pipes, as containers for highly toxic and radioactive materials and wastes and as fire resistant linings, pipe insulation and high temperature process pipes in many industries. The microporous structure of the ceramic hollow body provides high temperature stability.
Ceramic materials may be formed into hollow bodies by a variety of conventional processes such as dry pressing, wet extrusion, slip molding, isostatic pressing, hot pressing, and injection pressing. In the dry pressing processes a ground ceramic powder is dry-mixed with an organic binder, such as dextrin, and subjected to high pressures on the order of 1000 atmospheres inside steel molds. In wet extrusion processes the ceramic powder and binder are slurry-mixed and extruded through nozzles in a plastic consistnency.
Conventional processes require the hollow body to undergo high temperature sintering to achieve mechanically strong products. The sintering step is generally conducted in gas-fired tunnel furnaces or kilns at temperatures on the order of 1650.degree. C. to 1850.degree. C. This sintering process prevents cost effective manufacture of large diameter and/or long hollow bodies due to the prohibitive cost of the associated furnaces or kilns.
Another process for producing ceramic oxide hollow bodies is known as flame spraying as described in W. German Pat. No. 1,646,667. The ceramic oxide powder is atomized at high temperature resulting in a partial or complete change in its state of aggregation. The atomized particles are sprayed onto a rough surface of a solid substrate. This substrate acts as a binder. The particles bind to each other and with the substrate. This process presents disadvantages when thick-walled hollow bodies are required, because, as the ceramic oxide layers build up there is no longer any available surface area on the substrate to aid in bonding. As a result the outer layers tend to detach from the inner bound layers. In addition, due to the non-uniform temperature gradient between the substrate-ceramic layer and the purely ceramic layers internal cracks develop in the body. This leads to lower mechanical strength for the hollow body and increased permeability. The increase in permeability may result in leakage due to diffusion of gases or liquids from the interior through the hollow body. This process has not, therefore, been found to be effective when thick walled impermeable ceramic or ceramic oxide hollow bodies such as thick walled pipes are required.